Polymeric materials have many attractive properties, but their mechanical properties are insufficient for many structural applications. Fibre reinforced composites, wherein approximately 6 to 25 μm diameter fibres having high stiffness and/or strength, typically carbon, glass or aramide fibres, are embedded in a polymeric matrix have substantially higher mechanical properties allowing to reach very advantageous mechanical properties to density ratios. The reinforcing fibres may be short, long or continuous. When the mechanical properties of continuous fibre reinforced composites can be very high, the geometry of parts made of such composites is limited mostly to thin wall geometries. On the other hand, although the mechanical properties of short fibre reinforced composites are lower than the former, there is no limit to the geometry of part manufactured therewith, as they can be produced by extrusion or injection moulding.
When a fibre reinforced composite is submitted to a stress field, the stress is transferred from the matrix to the fibres through the matrix-fibre interface. If the latter is strong, the whole load is transferred to the fibre and the mechanical properties are high. If, on the other hand, the interfacial bond between matrix and fibres is low, a crack may initiate at and propagate along the fibre-matrix interface resulting in a premature failure. It is therefore very important to enhance the bond between matrix and fibres. The mechanical properties of short fibre reinforced composites are particularly sensitive to the interfacial strength between polymer matrix and fibres, because of the short size of the latter.
In order to allow handling of the fibres and to enhance interfacial adhesion with the matrix they are embedded in, the fibres are coated with a sizing which composition depends on the nature of the reinforcing fibre to be sized and on the matrix the fibres are to be used with. Glass fibres are usually sized with a silane based composition since Si—O—Si covalent bonds can be formed between, on the one hand, the glass fibre surface and silanols obtained by hydrolysing of the alkoxysilanes of the sizing and, on the other hand, between adjacent silanol groups, thus forming a crosslinked structure at the surface of the glass fibres. This crosslinked structure seems to enhance the fibre resistance to corrosion, typically to hydrolysis. Adhesion of the sizing with the matrix can be enhanced by the organic function of the silane coupling agent and by a film former, which nature depends on the matrix used. Sizing compositions usually comprise other additive such as a lubricant, antistatic agents, and the like. Numerous sizing compositions for glass fibres have been proposed in the art, as e.g., in JP2006016559, JP2006016558, JP2006016557, WO2004110948, US5877240, WO9818734, US4448910, WO200149627, WO200048957, WO200836224, US20070154697, WO200767667, US20070072989, WO200431246, WO200356095, WO200353875, US20060083922, WO200288044, WO200026155, WO9855415, WO9742129, CA2127015, EP-635462, US5389440, and are reviewed in E. P. Pluedemann, “Silane Coupling Agents”, Plenium Press (7982). Sizing compositions can be applied
Sizing compositions can be applied to the fibre surface at a single coating station or, can be formulated as a two-part composition to be applied at two coating stations. The latter technique permits to formulate sizing compositions combining components which are reactive with one another, which would be unsuitable for a one-station application process because of their limited shelf life. Two part sizing compositions are usually composed of a precursor applied first to the surface of the fibres, and of a binder applied thereafter. They are most suited for chopped fibres, wherein the precursor is applied to the fibre surface as they are drawn out of a bushing. The precursor coated fibres are then chopped and collected into a container wherein they are coated with the binder, usually by spraying thus forming fibre pellets. A thermal treatment is then applied to the fibre pellets to evaporate water and to react the components of the sizing composition with the glass fibre surface and with one another.
The sized fibre pellets can advantageously be blended with a thermoplastic polymer in the screw of an extruder, wherein the fibre pellets are disrupted and the length of the fibres further decreased while they are homogenously mixed with the polymer; An extruded profile, generally in the shape of a wire, is chopped to form composite pellets of the polymer loaded with short glass fibres. Said composite pellets can be melted and shaped into a final part by injection moulding or extrusion.
The present invention proposes sizing compositions for glass fibres which yield high mechanical properties measured both dry as moulded (DAM) and after ageing in moist conditions. The sizing is particularly effective with thermoplastic polyester matrices. This and other advantages of the present invention are presented in the following.